MAX is not the only option to achieve fuel efficiency. In fact, MAX is the failed answer from Boeing to the fuel efficiency problem.

Airbus has NEO:

https://en.wikipedia.org/wiki/Airbus_A320neo_family

"After a year in service, Lufthansa confirmed the 20% efficiency gain per passenger with up to 180 seats, along with reduced noise and CO₂ emissions.[26] Operators confirm the 15% per seat fuel-burn savings even counterbalanced by the added weight on short sectors, which can rise to 16–17% on longer routes and to 20% or more for Lufthansa with 180 passengers up from 168 with two more seat rows"